The invention relates to an optical arrangement, in particular to a microlithographic projection printing installation, in particular having a slot-shaped image field or rotationally non-symmetrical illumination, comprising a light source, which emits radiation, and a refractive optical element, which is heated by being acted upon in a rotationally non-symmetrical manner by the radiation, wherein at least one electric heating element is coupled to the optical element.
The imaging quality of such an optical arrangement is often impaired by rotationally non-symmetrical image defects. Such image defects arise, for example, not only as a result of rotationally non-symmetrical light-induced heating of the optical element but also as a result of other light-induced effects, such as e.g. compaction, which lead to a corresponding, rotationally non-symmetrical expansion and/or refractive index distribution in the optical element.
When high imaging quality is required, as it is in particular for microlithographic projection printing processes, the described light-induced image defects cannot be tolerated. From EP 0 678 768 A2 it is known to seek an improvement of the imaging properties by using additional heating to achieve a symmetrical and/or homogeneous temperature distribution. To said end, a plurality of heating elements are thermally coupled to the peripheral surface of a lens and therefore heat said lens from the direction of its edge.
Such heating of the lens has the drawback that the peripheral surface of the lens has to be heated to a relatively high extent in order, despite the generally poor heat-conducting properties of the lens material, to achieve such a symmetrical and/or homogeneous temperature distribution in thexe2x80x94for the imaging propertiesxe2x80x94mostly relevant central region of the lens. Intensive heating in the peripheral region of the lens leads however to the risk of the lens and/or lens mounting being damaged by thermal stresses.
Because of the relatively large distance between the peripheral region and the central region of the lens, moreover, a purposeful structured influencing of the temperature distribution in the vicinity of the central region is practically impossible.
The object of the present invention is therefore to develop an optical arrangement of the type described initially in such a way that the temperature distribution in the optical element may be rendered more symmetrical and/or homogeneous.
Said object is achieved according to the invention in that the heating element comprises: a resistance heating coating, which is carried by the optical element and which is substantially optically transparent in the region of the surface of the optical element acted upon by the radiation of the light source and comprises a plurality of parallel, electrically mutually insulated coating strips; as well as a heating current source.
A heating element having coating strips arranged in said manner enables good adaptation of the imaging properties by means of the overall temperature distribution which arises in the lens body on account of the additional heating. In particular, the central region of the lens body and the region surrounding it, the rotationally non-symmetrical heating of which is a primary cause of image defects of the lens as a result of residual absorption of the radiation of the light source, may be directly heated. As the coating strips are optically transparent, they do not needlessly limit the aperture of the lens. Thus, even when the surfaces of the lens body which are to be acted upon by the radiation of the light source change, the same heating element may be used.
The coating strips may form a layer which substantially covers the optical element particularly in the region surrounding the surface acted upon by the radiation of the light source, wherein only narrow gaps remain between the individual coating strips. Such an arrangement of the coating strips enables practically total heating of the lens surface particularly in the region surrounding the surface acted upon by the radiation of the light source. Thus, good control of the imaging properties is possible as a result of the temperature distribution in the optical element being adjustable by means of the heating. The narrow gaps between the coating strips serve as electric insulation between the latter.
The coating strips may carry an anti-reflecting coat at the side remote from the optical element. Said anti-reflecting coat is simultaneously a protective layer for the coating strips. Particularly in the case of coating strips made of a material having a refractive index greater than that of the material used to manufacture the optical element, increased reflection losses may be avoided at the boundary layer of the coating strips remote from the optical element.
The anti-reflecting coat is preferably a continuous layer on the optical element. Thus, for example, both the coating strips and the anti-reflecting coat may be applied in a simple manner by two successive vapour-deposition or other coating operations.
The coating strips are preferentially made of a material which in the region of the wavelength of the radiation has substantially the same refractive index as the material of the optical element. By said means, reflection losses are avoided at the boundary layer between the coating strips and the lens body.
At least two coating strips may have electrical resistances per unit length which differ from one another. With a given heating current, the heat output increases with the heating resistance. Through the preselection of appropriate resistances per unit length it is already possible, given a constant heating current, to achieve a heat output distribution over the lens surface which leads to a good symmetrical and/or homogeneous temperature distribution in the lens body. It is therefore possible to dispense with current control of the heating current.
When the coating strips comprise a plurality of regions of differing electrical resistance per unit length, an additional degree of freedom for adaptation of the heat output distribution over the surface of the lens body is provided, without a variation of the heating current being required for said purpose.
A continuous change of the electrical resistance per unit length may be effected along the coating strips. Such a continuous change leads to a correspondingly continuous variation of the heat energy delivered over the length of the coating strip. Undesirable discontinuities of the temperature distribution are therefore avoided.
In a preferred form of construction, the coating strips are disposed mirror-symmetrically relative to a meridional plane of symmetry, which lies parallel to the coating strips. As a result, given a radiated power distribution of the action by the light source which likewise has a mirror symmetry, the adaptation of the heat output to produce a symmetrical and/or homogeneous temperature distribution is facilitated.
When the coating strips, which are associated with one another mirror-symmetrically by virtue of the meridional plane of symmetry, have identical electrical resistances, manufacture of the heating element is simplified.
In a preferred form of construction of the invention, the heating current source comprises at least two electric supply devices, and the coating strips, which are associated with one another mirror-symmetrically by virtue of the meridional plane of symmetry, are heated by the same electric supply device. This simplifies control of the heating element.
The heating current source may have a communication link to a heating current control circuit, which preselects the outputs to be delivered by the electric supply devices. This enables automatic adaptation of the heat output distribution to the change of external operating parameters of the projecting printing installation.
The heating current control circuit preferentially has a communication link to a sensor, which measures the imaging properties of the optical element and/or optical arrangement, and the preselection of the outputs to be delivered to the electric supply devices is effected in dependence upon the measurement result of the sensor. Such an arrangement enables automatic adaptation of the heat output distribution to optimize the imaging properties of the optical element and/or entire optical arrangement which are acquired by the sensor.
The sensor may be a position-sensitive sensor, which is disposed in a focal plane of the optical element and/or optical arrangement. With such a sensor, precise acquisition of the imaging properties of the optical element and/or optical arrangement is possible.
The position-sensitive sensor may be a CCD detector. Such a sensor has high optical sensitivity and good linearity.